This happens only when you have "Print Background (colors & images)" turned
off... I think this was exposed by your patch in 191574. The default value
for aUsePrintSettings used to be false, but now it's getting called with true
in nsFrame. So it's bailing before the theme paints. The theme stuff should
probably be moved before the printing check.. see
http://lxr.mozilla.org/mozilla/source/layout/html/style/src/nsCSSRendering.cpp#2
836